Metal plating process



United States Patent 9 2,965,551 METAL PLATING PROCESS Henri Richaud, Chambery, France, assignor to Pechiney Compagnie de Produits Chimiques et Electrometallurgiques, Paris, France Filed Aug. 7, 1957, Ser. No. 676,722 Claims priority, application France Aug. 8, 1956 16 Claims. (Cl. 204-32) This invention relates to a process for preparing a metal surface for applying thereto a metallic coating.

It is an object of my invention to provide a process for the preparation of a metal surface prior to the application thereto of a metallic coating, which process permits to obtain a better adherence and improved decorative effects of the coating to be subsequently applied.

It is another object of my invention to provide improvements in the preparation of metal surfaces as a preliminary treatment to the subsequent coating of these surfaces with metallic coatings, so as to obtain coatings which will adhere better than the conventionally known ones, and which can be deposited, for instance, from baths free from undesirable components liable to contaminate the ultimate metallic deposits.

It is well known in the art of metal plating that, when the surface of a metal object is to be covered or coated with a metallic deposit with the aid of a chemical or electrolytical process, the metal surface must usually be suitably prepared in a preliminary treatment, so as to ensure the formation thereon of a well adherent coating.

This problem of obtaining a properly adherent coating is of particular importance, when chemical deposits are to be made on surfaces where the base metal is highly oxidizable. Often, the known methods of treating metal surfaces in order to improve the adherence of the coating to be applied, are detrimental to other properties of the coating, such as its outer appearance, its resistance to corrosion and the like. In certain cases, the formation of satisfactory chemical or electrolytical deposits may even be made impossible by such preliminary treatments, if the presence of an intermediary metal used in such treatment leads to the pollution of the plating bath containing the desired metal to be deposited.

These drawbacks are avoided, and the coating of a metal surface with a cover layer of another metal or other metals is greatly facilitated by the process according to my invention, which comprises the preliminary treatment of the metal surface to be coated prior to plating the same with the desired coating metal or metals by the step of electrolytic oxidation of that surface in the presence of at least one compound of that aforesaid coating metal or metals, and in a medium containing at least one reducing agent.

According to one important feature of my invention, the metal surface to becoated is first anodically oxidized and immersed in a solution which contains simultaneously a compound of the desired coating metal or metals, and a. reducing agent. It is preferred to have the base metal connected to the source of electric current at the moment of immersing the base, metal surface into the aforesaid solution, so that curent passes through the contact sur face from the very first instant of establishing contact between the metal surface and the solution constituting the oxidation bath.

This preferably brief step of oxidizing the metal surface to be coated is controlled in such a manner that a thin oxidic layer is formed on the metal surface to be coated. In most cases, the duration of this anodic oxidation step variesbetween a few seconds and up to one minute.

-; Lhave found it to be of advantage to control this oxidation step in such a manner that the oxidic layer is of a thickness below 20 microns, and preferably between 0.1 and 10 microns. Especially favorable results are obtained if oxide layers are allowed to form the thickness of which varies between 0.2 and 2 microns, and, in the case of aluminum and its alloys, more particularly between 0.5 and 1 micron.

This anodic oxidation step according to my invention can be carried out with the aid of different types of electric current such as direct, alternating or undulatory current at carefully chosen rates of current density and voltage.

The oxidizing step in the process according to my invention is preferably preceded by conventional cleansing steps such as mechanical polishing, degreasing with organic agents such as trichloroethylene, and/or cathodic cleaning followed by rinsing with distilled water.

Prior to carrying out the step of anodic oxidation, an oxidation bath is prepared, which comprises at least one compound of the desired coating metal or metals and at least one reducing agent, and the pH value of this oxidation bath is adjusted, by a conventional addition of an acidic or basic agent, to a determined optimal pH, which is, by way of example only in the range of preferably 0.5 to 4.8 in the case of aluminum alloys to be coated with nickel.

The process according to the invention can be used in a variety of applications in the field of metal plating, and particularly in coating metal surfaces with protective, well adherent cover layers of chromium, copper, cadmium, nickel, silver, gold, zinc and others. These coating metals may be applied, in acidic or alkaline baths, as the case may be, to a great variety of suitable base metals such as: aluminum, magnesium, titanium, uranium, beryllium, zirconium, iron, copper, zinc, nickel, and their different alloys. Among these, there are steel, cast and pig iron, brass, the various bronzes, duralumin and other aluminum alloys.

I have found that the process according to my inven-- tion gives satisfactory results in all combination of one of the above named base metals with a coating metal as mentioned. It is particularly useful in coating easily oxidizable metals such as, for instance, aluminum, magnesium, titanium, uranium, beryllium, iron, zirconium, and the like metals and their alloys.

The bath in which the anodic oxidation step according to my invention is to be carried out, should contain simultaneously, at least one compound of the coating metal or metals, and a reducing agent. While it is preferable to have present in the solution a cation of the desired metal, it is also possible to use a solution containing a compound of the coating metal which is little or completely un-ionized. Therefore, salts of the coating metal or metals, such as sulfates, chlorides, fluorides, perchlorates, acetates and the like are suitable, but so are cyano complexes, tartrates and different other organic compounds of the coating metal or metals, contained singly or in combination with each other, in the oxidation bath.

. Reducing agents in the oxidation bath which are useful in carrying out my invention in practice, can be of Widely varying nature, depending on the nature of base metals and on the coating metals to be applied thereto. Thus, reducing salts such as sulfites, thiosulfates, hydrosulfites, arsenites, phosphites or hypophosphites, preferably in the form of their sodium or potassium salts, as well as the salts of metals of less than their maximum valency according to the periodic table can be used. In the latter category, ferrous, dior trivalent titanous, cu prous, stannous and the like salts are suitable.

By selecting a salt of inferior valency stage of the coating metal, where such salt is available, it is possible to provide in'the oxidation bath one and the same coin.-

3.. ponent constituting, at the same time, the reducing agent and the donor of cations of the coating metal.

Furthermore, organic reducing agents may be used either alone or in mixture with eachother and/or with the -inorganic"- reducing agents described above. Suitable organic reducing'agents are, for instance, alcohols such as methanol, aldehydes, particularlyformaldehyde, ketones, organic acids in. particular of the hydroxy mono or dicarboxylic type such as lactic or succinic acid either in the form of the free acid or a sodium or potassium salt thereof. I

@Hydrazine, hydroxylamine andsimilar compounds are equally suitable as reducingagents'. for the purpose of my invention. 7

It is often of an advantage to effect the anodic oxidation in a bath .havinga composition similar to the one in which the final step. of depositing the desired metal coating is to'be carried out, while the pH value, rH value (redoxy value), the concentration and the temperature of the bath are selected to be particularly suitable for the anodic oxidation of the base metal surface.

If the presence of a reducing agent required during the anodic oxidation step according to the present invention has no disturbing effects during the ultimate chemical or electrochemical deposition of thedesired metal coating, the anodic oxidation step may be carried out directly in the same bath that is subsequently-used for obtaining the coating metal deposit.

The process according to the invention is further illustrated by a numberofexamples given below which are, however, not meant to be-limitative in any way:

Example] In orderto improve the adhesion to the base metal surface of a nickel"coatings formed lay-chemical deposition' on a duralumin surface, awell knownaluminum alloy, thefollowing treatment is'followed:

The surface to be coated of an object of duralumin is polished mechanically in a conventional manner and degreased first with the aid of trichloroethylene, and then cathodically with an aqueous solution of 100 grams/liter of NaPO 2O grams/liter of Na CO and 5 grams/liter of Na SiO at 12 volts and 5 amperes/square decimeter during two minutes. After rinsing, neutralization with nitric acid and renewed rinsing, the surface of the duralumin object to be coated is immersed in an oxidation bath of thefollowing composition (in grams/liter'=g./l.):

G./l. Crystalline nickel sulfateNiSO .7H O 23 Sodium hypophosphite Na PO 24 Lactic acid 27 Sodium. succinate 20 the pH of which bath is adjusted to 4.7 by conventional addition of either H 804 or NaOH.

The duralumin object'is connected prior to immersion to the positive pole of a DC. source, and concurrently with its immersion in the aforesaid oxidation bath, a current of 14 volts' is caused to pass therethrough at a current density of 1.7 amperes per square decimeter treated surface, during 30' seconds. The object is coated with oxide layer having a thickness of about 0.8 micron. The temperature of the bath during the oxidation step is about 96 C.

After the'current has'been interrupted, the object re. mains'in the'same bath during approximately one hour at a temperature of 96 to 98 C. Thereby a desired.

nickel coating having: a thickness of about 22 microns is deposited on the anodically oxidized surface of the ob-' ject. The coated object is then rinsed and dried first in air and then:in an oven at '165 C. for one hour.

Conventional-tests to -which the coated object is'subjected-reveal that the nickel layer applied by the process according. to'this" invention adheres' much better to. the base metal. thaw-that er. anyyknown. .processs Example II pole of a direct current source and immersed in an oxidation bath containing the same components as in Example I, but the pH value of which has been adjusted to one by a corresponding addition of H Current is" then'passed through' the objectand'bath during 30 seconds at 12 volts and with a current density of 10 amperes per' square decimeter of the'object surface to be oxidized. The bath temperature is C.

After the current has been interrupted, the object is transferred," without rinsing, to an identical bath as described in Example I '(the pH 'value of which has been adjusted to 4.7). The object'remains in this bath for two hours at a temperature of 96 to 98 C. wherebya nickel layer of a thickness of'44'microns is deposited thereon.

After rinsing and drying in an oven for about one hour. at 165 C., the coating reveals the same advantageous properties asthe' one obtained in Example 1.

Example III Plates'of aluminum of 99.5% purity"are' degreased with trichloroethylene, cleaned "duringone minute at'a" temperature of 70 C. in a solution of caustic soda'hav-- ing aconcentration' of g./1., rinsed, and neutralized,

by treatment at room temperature during a few seconds,"

with nitric acidof 40 B.

After renewed rinsing, the plates are'connected to the" positive terminal of a source of 'direct electric current:

and dipped into an aqueous'oxidation'bath having the following composition:

G./l." NiSO -7I-I O 23. HCHO (formaldehyde) 9" Lacticacid 27' Na-succinate' 20" The pH value of the bath is adjusted to one'(pH=1),

and the bath is heated to 95 C., whereupon a current,

of 14 volts and a density of 10 amperes per square decim eter treated surface is passed therethrough during 30 seconds. After'the current has been interrupted, the plates are transferred, without rinsing, to a nickel-plating bath similar to that described in Example I. The plates remain in this bath for one hour at a temperature of 96 to 98 0., whereby nickel layers of a thickness of 22 microns and excellent adherence to the base metal are.

obtained on the plates.

Example IV The same treatment as described in Example llliis carried out with an oxidation bath containing 10 g./l

of ethyl alcohol instead'of formaldehyde in the bath solution.

Example V In the same 'treatment 'as describedin Example 111,

formaldehyde is'replaced'in' the oxidation bath by 56- g./l. of neutral, crystalline sodium sulfite, Na SO -7H O' the-oxidation step istcarried outwith the samecurrent density of lflwamperes per square decimeter'oftreated surface, but at a. highervoltage of 18' volts;

It will be understood that while there have :beenflgivefii hereinzcertainispecific examples I of the praetice of this invention; it isznotrintendedi-thereby to havezthis invenw tion limited to or circumscribed by the specific details of materials, proportions or conditions herein specified, in view of the fact that this invention may be modified according to individual preference or conditions Without necessarily departing from the spirit of this disclosure and the scope of the appended claims.

What I claim is:

1. A process for coating a metal surface with nickel, comprising the steps of (a) degreasing and cleaning the surface in a conventional manner, and (b) subjecting the cleaned surface to electrolytic oxidation in a bath capable of providing an oxidizing environment at the anode, said bath containing at least one compound of nickel and at least one reducing agent being a mineral salt selected from the group consisting of sulfites, thiosulfates, hydrosulfites, arsenites, phosphites, and hypophosphites, and (c) coating the oxidic layer thus formed on said surface in a coating bath with a cover layer of nickel.

2. A process as described in claim 1, characterized in that the bath in which said electrolytic oxidation is carried out, comprises an aqueous solution of a salt of nickel.

3. A process as described in claim 1, characterized in that the bath in which said electrolytic oxidation is carried out, com rises an aqueous solution of a complex compound of nickel.

4. A process for coating the metal surface of a body consisting of a metal with nickel, comprising the steps of (a) connecting said body in an electrolytic circuit, (b) immersing said body surface into an electrolytic bath capable of providing an oxidizing environment at the anode and containing at least one compound of nickel and at least one reducing agent being a mineral salt from the group consisting of sulfites, thiosulfates, hydrosulfites, arsenites, phosphites, and hypophosphites, (c) electrolytically oxidizing said surface by passing current through said circuit, and (d) coating the oxidic layer thus formed on said surface in a coating bath with a cover layer of nickel.

5. The process as described in claim 4 characterized in that said current passed through said circuit is a direct current.

6. The process as described in claim 4 characterized in that said current passed through said circuit is an alternating current.

7. The process as described in claim 4 characterized in that said current passed through said circuit is an undulatory current.

8. The process as described in claim 4 characterized in that the duration of electrolytically oxidizing said surface varies from a few seconds to one minute.

9. A process for coating the metal surface of a body consisting of a metal with nickel, comprising the steps of (a) connecting said body in an electrolytic circuit, (b) immersing said body surface into an electrolytic bath capable of providing an oxidizing environment at the anode and containing at least one compound of nickel and at least one reducing agent being a mineral salt from the group consisting of sulfites, thiosulfates, hydrosulfites, arsenites, phosphites, and hypophosphites, (c) electrolytically oxidizing said surface by passing current through said circuit until an oxidic coating is formed on said body surface which coating is less than 20 microns thick, and (d) coating the oxidic layer thus formed on said surface in a coating bath with a cover layer of nickel.

10. A process for coating the metal surface of a body consisting of a metal with nickel, comprising the steps of (a) connecting said body in an electrolytic circuit, (b) immersing said body surface into an electrolytic bath capable of providing an oxidizing environment at the anode and containing at least one compound of nickel and at least one reducing agent being a mineral salt from the group consisting of sulfites, thiosulfates, hydrosulfites, arsenites, phosphites, and hypophosphites, (c) electrolytically oxidizing said surface by passing current through said circuit until an oxidic coating is formed on said body surface which coating has a thickness between 0.1 and 10 microns, and (d) coating the oxidic layer thus formed on said surface in a coating bath with a cover layer of nickel.

11. The process as described in claim 4, characterized in that said reducing agent is a salt of a metal in an inferior valency stage below its full valency according to the Periodic Table.

12. The process as described in claim 4, characterized in that said reducing agent is a salt of a metal in an inferior valency stage below its full valency according to the Periodic Table, selected from the group consisting of the salts of divalent iron, divalent and trivalent titanium, divalent tin, and monovalent copper.

13. A process for plating the metal surface of a body consisting of an easily oxidizable base metal with nickel, comprising the steps of successively (a) connecting said body to the anode of the current source of an electrolytic circuit, (b) immersing said metal surface of said body into an aqueous electrolytic bath capable of providing an oxidizing environment at the anode and containing at least one compound of nickel and at least one reducing agent being a mineral salt selected from the group consisting of sulfites, thiosulfates, hydrosulfites, arsenites, phosphites, and hypophosphites, (c) anodically oxidizing said surface by passing current through said circuit so as to deposit an oxidic layer on said surface, and (d) coating the oxidic layer on said surface with a cover layer of nickel.

14. A process as described in claim 13, characterized in that said easily oxidizable metal is a metal selected from the group consisting of aluminum, magnesium, titanium, uranium, zirconium, iron and their alloys.

15. A process for plating the metal surface of a body consisting of an easily oxidizable base metal with nickel, comprising the steps of successively (a) connecting said body to the anode of the current source of an electrolytic circuit, (b) immersing said metal surface of said body into an aqueous electrolytic bath capable of providing an oxidizing environment at the anode and containing at least one compound of nickel and at least one reducing agent being a mineral salt selected from the group consisting of sulfites, thiosulfates, hydrosulfites, arsenites, phosphites, and hypophosphites, (c) anodically oxidizing said surface by passing current through said circuit so as to deposit an oxidic layer on said surface, and (d) coating the oxidic layer on said surface in an electrolytic nickel plating bath having a higher pH value than said reducing agent-containing bath with a cover layer of nickel.

16. A process for the preliminary treatment of the surface of a body of a metal selected from the group consisting of aluminum and aluminum alloys, prior to the plating of said surface with nickel, comprising the steps of (a) connecting said body to the anode of a current source in an electrolytic circuit, (b) immersing said surface into an aqueous bath capable of providing an oxidizing environment at the anode and containing nickel sulfate, an alkali metal hypophosphite, and at least one salt of an organic acid, (0) electrolytically oxidizing said surface by passing current through said circuit until an oxidic coating is formed on said body surface, which coating has a thickness between 0.1 and 10 microns.

References Cited in the file of this patent UNITED STATES PATENTS 1,947,981 Fischer Feb. 20, 1934 2,231,086 Muller et a1 Feb. 11, 1941 2,473,163 McCoy June 14, 1949 2,495,941 Mondolfo Jan. 31, 1950 FOREIGN PATENTS 515,648 Great Britain Dec. 11, 1939 

1. A PROCESS FOR COATING A METAL SURFACE WITH NICKEL, COMPRISING THE STEPS OF (A) DECREASING AND CLEANING THE SURFACE IN A CONVENTIONAL MANNER, AND (B) SUBJECTING THE CLEANED SURFACE TO ELECTROLYTIC OXIDATION IN A BATH CAPABLE OF PROVIDING AN OXIDIZING ENVIRONMENT AT THE ANODE, SAID BATH CONTAINING AT LEAST ONE COMPOUND OF NICKEL AND AT LEAST ONE REDUCING AGENT BEING A MINERAL SALT SELECTED FROM THE GROUP CONSISTING OF SULFITES, THIOSULFATES, HYDROSULFITES, ARSENITES, PHOSPHITES, AND HYPOPHOSPHITES, AND (C) COATING THE OXIDIC LAYER THUS FORMED ON SAID SURFACE IN COATING BATH WITH A COVER LAYER OF NICKEL. 